I have a virtual server with a static IP and a real FQDN. I set up Postfix on it and everything works. While setting up Postfix, the guide that I followed suggested that I enter the server's FQDN as Postfix system mail name which I did (My FQDN and reverse DNS given to me by my hosting company is something like virtualhost123.myhostingprovider.com).

Now I want to host two domains on this server: say domainA.com and domainB.com. When I send outgoing mail to Gmail from my server for example (let's say from e-mail address support@domainA.com) everything works fine but in the e-mail received, Gmail says:

E-mail from support@domainA.com via virtualhost123.myhostingprovider.com.

I am worried that e-mail providers could block me thinking I'm just relaying spam (I am not sending spam but since the sender e-mail domain and via domain don't match maybe it looks suspicious).

Should I be worried? What should Postfix system mail name be set to for a server that hosts multiple domains?

What are the best practices about this or stuff that I need to do so big e-mail providers like Gmail, Hotmail, etc. don't block my server?